/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.*;
import java.util.concurrent.atomic.AtomicInteger;
import java.security.AccessControlContext;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.security.PrivilegedExceptionAction;
import java.security.PrivilegedActionException;
import java.security.AccessControlException;
import sun.security.util.SecurityConstants;

/**
 * Factory and utility methods for {@link Executor}, {@link ExecutorService},
 * {@link ScheduledExecutorService}, {@link ThreadFactory}, and {@link Callable}
 * classes defined in this package. This class supports the following kinds of
 * methods:
 * 
 * 在这个包里为Executor，ExecutorService，ScheduledExecutorService，ThreadFactory，Callable类定义了工厂和实用的方法。
 * 此类支持以下各种方法； 创建并返回一个常用的配置ExecutorService的方法。
 * 创建并返回一个常用的配置ScheduledExecutorService的方法。
 * 创建并返回一个包装ExecutorService，通过使特定实现方法的不可访问，禁用重新配置。
 * 创建并返回一个ThreadFactory，可以将新创建的线程设置为已知状态。 
 * 创建并返回一个非闭包形式的Callable的方法，这样可将其用于需要 Callable 的执行方法中。
 * <ul>
 * <li>Methods that create and return an {@link ExecutorService} set up with
 * commonly useful configuration settings.
 * <li>Methods that create and return a {@link ScheduledExecutorService} set up
 * with commonly useful configuration settings.
 * <li>Methods that create and return a "wrapped" ExecutorService, that disables
 * reconfiguration by making implementation-specific methods inaccessible.
 * <li>Methods that create and return a {@link ThreadFactory} that sets newly
 * created threads to a known state.
 * <li>Methods that create and return a {@link Callable} out of other
 * closure-like forms, so they can be used in execution methods requiring
 * {@code Callable}.
 * </ul>
 *
 * @since 1.5
 * @author Doug Lea
 */
public class Executors {

    /**
     * Creates a thread pool that reuses a fixed number of threads operating off a
     * shared unbounded queue. At any point, at most {@code nThreads} threads will
     * be active processing tasks. If additional tasks are submitted when all
     * threads are active, they will wait in the queue until a thread is available.
     * If any thread terminates due to a failure during execution prior to shutdown,
     * a new one will take its place if needed to execute subsequent tasks. The
     * threads in the pool will exist until it is explicitly
     * {@link ExecutorService#shutdown shutdown}.
     *
     * @param nThreads the number of threads in the pool
     * @return the newly created thread pool
     * @throws IllegalArgumentException if {@code nThreads <= 0}
     */
    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>());
    }

    /**
     * Creates a thread pool that maintains enough threads to support the given
     * parallelism level, and may use multiple queues to reduce contention. The
     * parallelism level corresponds to the maximum number of threads actively
     * engaged in, or available to engage in, task processing. The actual number of
     * threads may grow and shrink dynamically. A work-stealing pool makes no
     * guarantees about the order in which submitted tasks are executed.
     *
     * @param parallelism the targeted parallelism level
     * @return the newly created thread pool
     * @throws IllegalArgumentException if {@code parallelism <= 0}
     * @since 1.8
     */
    public static ExecutorService newWorkStealingPool(int parallelism) {
        return new ForkJoinPool(parallelism, ForkJoinPool.defaultForkJoinWorkerThreadFactory, null, true);
    }

    /**
     * Creates a work-stealing thread pool using all
     * {@link Runtime#availableProcessors available processors} as its target
     * parallelism level.
     * 
     * @return the newly created thread pool
     * @see #newWorkStealingPool(int)
     * @since 1.8
     */
    public static ExecutorService newWorkStealingPool() {
        return new ForkJoinPool(Runtime.getRuntime().availableProcessors(),
                ForkJoinPool.defaultForkJoinWorkerThreadFactory, null, true);
    }

    /**
     * Creates a thread pool that reuses a fixed number of threads operating off a
     * shared unbounded queue, using the provided ThreadFactory to create new
     * threads when needed. At any point, at most {@code nThreads} threads will be
     * active processing tasks. If additional tasks are submitted when all threads
     * are active, they will wait in the queue until a thread is available. If any
     * thread terminates due to a failure during execution prior to shutdown, a new
     * one will take its place if needed to execute subsequent tasks. The threads in
     * the pool will exist until it is explicitly {@link ExecutorService#shutdown
     * shutdown}.
     *
     * @param nThreads      the number of threads in the pool
     * @param threadFactory the factory to use when creating new threads
     * @return the newly created thread pool
     * @throws NullPointerException     if threadFactory is null
     * @throws IllegalArgumentException if {@code nThreads <= 0}
     */
    public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
        return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>(), threadFactory);
    }

    /**
     * Creates an Executor that uses a single worker thread operating off an
     * unbounded queue. (Note however that if this single thread terminates due to a
     * failure during execution prior to shutdown, a new one will take its place if
     * needed to execute subsequent tasks.) Tasks are guaranteed to execute
     * sequentially, and no more than one task will be active at any given time.
     * Unlike the otherwise equivalent {@code newFixedThreadPool(1)} the returned
     * executor is guaranteed not to be reconfigurable to use additional threads.
     *
     * @return the newly created single-threaded Executor
     */
    public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService(
                new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()));
    }

    /**
     * Creates an Executor that uses a single worker thread operating off an
     * unbounded queue, and uses the provided ThreadFactory to create a new thread
     * when needed. Unlike the otherwise equivalent
     * {@code newFixedThreadPool(1, threadFactory)} the returned executor is
     * guaranteed not to be reconfigurable to use additional threads.
     *
     * @param threadFactory the factory to use when creating new threads
     *
     * @return the newly created single-threaded Executor
     * @throws NullPointerException if threadFactory is null
     */
    public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
        return new FinalizableDelegatedExecutorService(new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>(), threadFactory));
    }

    /**
     * Creates a thread pool that creates new threads as needed, but will reuse
     * previously constructed threads when they are available. These pools will
     * typically improve the performance of programs that execute many short-lived
     * asynchronous tasks. Calls to {@code execute} will reuse previously
     * constructed threads if available. If no existing thread is available, a new
     * thread will be created and added to the pool. Threads that have not been used
     * for sixty seconds are terminated and removed from the cache. Thus, a pool
     * that remains idle for long enough will not consume any resources. Note that
     * pools with similar properties but different details (for example, timeout
     * parameters) may be created using {@link ThreadPoolExecutor} constructors.
     *
     * 创建一个线程池，该线程池根据需要创建新线程，但是将在先前构造的线程可用时重用它们。
     * 这些线程池通常将提高执行许多短期异步任务的程序的性能。
     * 调用{@code execute}将重用以前构造的线程（如果有）。
     * 如果没有可用的现有线程，则将创建一个新线程并将其添加到池中。
     * 六十秒内未使用的线程将终止并从缓存中删除。
     * 因此，保持空闲时间足够长的线程池不会消耗任何资源。
     * 请注意，可以使用{@link ThreadPoolExecutor}构造函数创建具有相似属性但细节不同（例如，超时参数）的线程池。
     * @return the newly created thread pool
     */
    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>());
    }

    /**
     * Creates a thread pool that creates new threads as needed, but will reuse
     * previously constructed threads when they are available, and uses the provided
     * ThreadFactory to create new threads when needed.
     * 
     * @param threadFactory the factory to use when creating new threads
     * @return the newly created thread pool
     * @throws NullPointerException if threadFactory is null
     */
    public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>(),
                threadFactory);
    }

    /**
     * Creates a single-threaded executor that can schedule commands to run after a
     * given delay, or to execute periodically. (Note however that if this single
     * thread terminates due to a failure during execution prior to shutdown, a new
     * one will take its place if needed to execute subsequent tasks.) Tasks are
     * guaranteed to execute sequentially, and no more than one task will be active
     * at any given time. Unlike the otherwise equivalent
     * {@code newScheduledThreadPool(1)} the returned executor is guaranteed not to
     * be reconfigurable to use additional threads.
     * 
     * @return the newly created scheduled executor
     */
    public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
        return new DelegatedScheduledExecutorService(new ScheduledThreadPoolExecutor(1));
    }

    /**
     * Creates a single-threaded executor that can schedule commands to run after a
     * given delay, or to execute periodically. (Note however that if this single
     * thread terminates due to a failure during execution prior to shutdown, a new
     * one will take its place if needed to execute subsequent tasks.) Tasks are
     * guaranteed to execute sequentially, and no more than one task will be active
     * at any given time. Unlike the otherwise equivalent
     * {@code newScheduledThreadPool(1, threadFactory)} the returned executor is
     * guaranteed not to be reconfigurable to use additional threads.
     * 
     * @param threadFactory the factory to use when creating new threads
     * @return a newly created scheduled executor
     * @throws NullPointerException if threadFactory is null
     */
    public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {
        return new DelegatedScheduledExecutorService(new ScheduledThreadPoolExecutor(1, threadFactory));
    }

    /**
     * Creates a thread pool that can schedule commands to run after a given delay,
     * or to execute periodically.
     * 
     * @param corePoolSize the number of threads to keep in the pool, even if they
     *                     are idle
     * @return a newly created scheduled thread pool
     * @throws IllegalArgumentException if {@code corePoolSize < 0}
     */
    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }

    /**
     * Creates a thread pool that can schedule commands to run after a given delay,
     * or to execute periodically.
     * 
     * @param corePoolSize  the number of threads to keep in the pool, even if they
     *                      are idle
     * @param threadFactory the factory to use when the executor creates a new
     *                      thread
     * @return a newly created scheduled thread pool
     * @throws IllegalArgumentException if {@code corePoolSize < 0}
     * @throws NullPointerException     if threadFactory is null
     */
    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize, ThreadFactory threadFactory) {
        return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
    }

    /**
     * Returns an object that delegates all defined {@link ExecutorService} methods
     * to the given executor, but not any other methods that might otherwise be
     * accessible using casts. This provides a way to safely "freeze" configuration
     * and disallow tuning of a given concrete implementation.
     * 
     * @param executor the underlying implementation
     * @return an {@code ExecutorService} instance
     * @throws NullPointerException if executor null
     */
    public static ExecutorService unconfigurableExecutorService(ExecutorService executor) {
        if (executor == null)
            throw new NullPointerException();
        return new DelegatedExecutorService(executor);
    }

    /**
     * Returns an object that delegates all defined {@link ScheduledExecutorService}
     * methods to the given executor, but not any other methods that might otherwise
     * be accessible using casts. This provides a way to safely "freeze"
     * configuration and disallow tuning of a given concrete implementation.
     * 
     * @param executor the underlying implementation
     * @return a {@code ScheduledExecutorService} instance
     * @throws NullPointerException if executor null
     */
    public static ScheduledExecutorService unconfigurableScheduledExecutorService(ScheduledExecutorService executor) {
        if (executor == null)
            throw new NullPointerException();
        return new DelegatedScheduledExecutorService(executor);
    }

    /**
     * Returns a default thread factory used to create new threads. This factory
     * creates all new threads used by an Executor in the same {@link ThreadGroup}.
     * If there is a {@link java.lang.SecurityManager}, it uses the group of
     * {@link System#getSecurityManager}, else the group of the thread invoking this
     * {@code defaultThreadFactory} method. Each new thread is created as a
     * non-daemon thread with priority set to the smaller of
     * {@code Thread.NORM_PRIORITY} and the maximum priority permitted in the thread
     * group. New threads have names accessible via {@link Thread#getName} of
     * <em>pool-N-thread-M</em>, where <em>N</em> is the sequence number of this
     * factory, and <em>M</em> is the sequence number of the thread created by this
     * factory.
     * 
     * @return a thread factory
     */
    public static ThreadFactory defaultThreadFactory() {
        return new DefaultThreadFactory();
    }

    /**
     * Returns a thread factory used to create new threads that have the same
     * permissions as the current thread. This factory creates threads with the same
     * settings as {@link Executors#defaultThreadFactory}, additionally setting the
     * AccessControlContext and contextClassLoader of new threads to be the same as
     * the thread invoking this {@code privilegedThreadFactory} method. A new
     * {@code privilegedThreadFactory} can be created within an
     * {@link AccessController#doPrivileged AccessController.doPrivileged} action
     * setting the current thread's access control context to create threads with
     * the selected permission settings holding within that action.
     *
     * <p>
     * Note that while tasks running within such threads will have the same access
     * control and class loader settings as the current thread, they need not have
     * the same {@link java.lang.ThreadLocal} or
     * {@link java.lang.InheritableThreadLocal} values. If necessary, particular
     * values of thread locals can be set or reset before any task runs in
     * {@link ThreadPoolExecutor} subclasses using
     * {@link ThreadPoolExecutor#beforeExecute(Thread, Runnable)}. Also, if it is
     * necessary to initialize worker threads to have the same
     * InheritableThreadLocal settings as some other designated thread, you can
     * create a custom ThreadFactory in which that thread waits for and services
     * requests to create others that will inherit its values.
     *
     * @return a thread factory
     * @throws AccessControlException if the current access control context does not
     *                                have permission to both get and set context
     *                                class loader
     */
    public static ThreadFactory privilegedThreadFactory() {
        return new PrivilegedThreadFactory();
    }

    /**
     * Returns a {@link Callable} object that, when called, runs the given task and
     * returns the given result. This can be useful when applying methods requiring
     * a {@code Callable} to an otherwise resultless action.
     * 
     * @param task   the task to run
     * @param result the result to return
     * @param <T>    the type of the result
     * @return a callable object
     * @throws NullPointerException if task null
     */
    public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }

    /**
     * Returns a {@link Callable} object that, when called, runs the given task and
     * returns {@code null}.
     * 
     * @param task the task to run
     * @return a callable object
     * @throws NullPointerException if task null
     */
    public static Callable<Object> callable(Runnable task) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<Object>(task, null);
    }

    /**
     * Returns a {@link Callable} object that, when called, runs the given
     * privileged action and returns its result.
     * 
     * @param action the privileged action to run
     * @return a callable object
     * @throws NullPointerException if action null
     */
    public static Callable<Object> callable(final PrivilegedAction<?> action) {
        if (action == null)
            throw new NullPointerException();
        return new Callable<Object>() {
            public Object call() {
                return action.run();
            }
        };
    }

    /**
     * Returns a {@link Callable} object that, when called, runs the given
     * privileged exception action and returns its result.
     * 
     * @param action the privileged exception action to run
     * @return a callable object
     * @throws NullPointerException if action null
     */
    public static Callable<Object> callable(final PrivilegedExceptionAction<?> action) {
        if (action == null)
            throw new NullPointerException();
        return new Callable<Object>() {
            public Object call() throws Exception {
                return action.run();
            }
        };
    }

    /**
     * Returns a {@link Callable} object that will, when called, execute the given
     * {@code callable} under the current access control context. This method should
     * normally be invoked within an {@link AccessController#doPrivileged
     * AccessController.doPrivileged} action to create callables that will, if
     * possible, execute under the selected permission settings holding within that
     * action; or if not possible, throw an associated
     * {@link AccessControlException}.
     * 
     * @param callable the underlying task
     * @param <T>      the type of the callable's result
     * @return a callable object
     * @throws NullPointerException if callable null
     */
    public static <T> Callable<T> privilegedCallable(Callable<T> callable) {
        if (callable == null)
            throw new NullPointerException();
        return new PrivilegedCallable<T>(callable);
    }

    /**
     * Returns a {@link Callable} object that will, when called, execute the given
     * {@code callable} under the current access control context, with the current
     * context class loader as the context class loader. This method should normally
     * be invoked within an {@link AccessController#doPrivileged
     * AccessController.doPrivileged} action to create callables that will, if
     * possible, execute under the selected permission settings holding within that
     * action; or if not possible, throw an associated
     * {@link AccessControlException}.
     *
     * @param callable the underlying task
     * @param <T>      the type of the callable's result
     * @return a callable object
     * @throws NullPointerException   if callable null
     * @throws AccessControlException if the current access control context does not
     *                                have permission to both set and get context
     *                                class loader
     */
    public static <T> Callable<T> privilegedCallableUsingCurrentClassLoader(Callable<T> callable) {
        if (callable == null)
            throw new NullPointerException();
        return new PrivilegedCallableUsingCurrentClassLoader<T>(callable);
    }

    // Non-public classes supporting the public methods

    /**
     * A callable that runs given task and returns given result
     */
    static final class RunnableAdapter<T> implements Callable<T> {
        final Runnable task;
        final T result;

        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }

        public T call() {
            task.run();
            return result;
        }
    }

    /**
     * A callable that runs under established access control settings
     */
    static final class PrivilegedCallable<T> implements Callable<T> {
        private final Callable<T> task;
        private final AccessControlContext acc;

        PrivilegedCallable(Callable<T> task) {
            this.task = task;
            this.acc = AccessController.getContext();
        }

        public T call() throws Exception {
            try {
                return AccessController.doPrivileged(new PrivilegedExceptionAction<T>() {
                    public T run() throws Exception {
                        return task.call();
                    }
                }, acc);
            } catch (PrivilegedActionException e) {
                throw e.getException();
            }
        }
    }

    /**
     * A callable that runs under established access control settings and current
     * ClassLoader
     */
    static final class PrivilegedCallableUsingCurrentClassLoader<T> implements Callable<T> {
        private final Callable<T> task;
        private final AccessControlContext acc;
        private final ClassLoader ccl;

        PrivilegedCallableUsingCurrentClassLoader(Callable<T> task) {
            SecurityManager sm = System.getSecurityManager();
            if (sm != null) {
                // Calls to getContextClassLoader from this class
                // never trigger a security check, but we check
                // whether our callers have this permission anyways.
                sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);

                // Whether setContextClassLoader turns out to be necessary
                // or not, we fail fast if permission is not available.
                sm.checkPermission(new RuntimePermission("setContextClassLoader"));
            }
            this.task = task;
            this.acc = AccessController.getContext();
            this.ccl = Thread.currentThread().getContextClassLoader();
        }

        public T call() throws Exception {
            try {
                return AccessController.doPrivileged(new PrivilegedExceptionAction<T>() {
                    public T run() throws Exception {
                        Thread t = Thread.currentThread();
                        ClassLoader cl = t.getContextClassLoader();
                        if (ccl == cl) {
                            return task.call();
                        } else {
                            t.setContextClassLoader(ccl);
                            try {
                                return task.call();
                            } finally {
                                t.setContextClassLoader(cl);
                            }
                        }
                    }
                }, acc);
            } catch (PrivilegedActionException e) {
                throw e.getException();
            }
        }
    }

    /**
     * The default thread factory
     */
    static class DefaultThreadFactory implements ThreadFactory {
        private static final AtomicInteger poolNumber = new AtomicInteger(1);
        private final ThreadGroup group;
        private final AtomicInteger threadNumber = new AtomicInteger(1);
        private final String namePrefix;

        DefaultThreadFactory() {
            SecurityManager s = System.getSecurityManager();
            group = (s != null) ? s.getThreadGroup() : Thread.currentThread().getThreadGroup();
            namePrefix = "pool-" + poolNumber.getAndIncrement() + "-thread-";
        }

        public Thread newThread(Runnable r) {
            Thread t = new Thread(group, r, namePrefix + threadNumber.getAndIncrement(), 0);
            if (t.isDaemon())
                t.setDaemon(false);
            if (t.getPriority() != Thread.NORM_PRIORITY)
                t.setPriority(Thread.NORM_PRIORITY);
            return t;
        }
    }

    /**
     * Thread factory capturing access control context and class loader
     */
    static class PrivilegedThreadFactory extends DefaultThreadFactory {
        private final AccessControlContext acc;
        private final ClassLoader ccl;

        PrivilegedThreadFactory() {
            super();
            SecurityManager sm = System.getSecurityManager();
            if (sm != null) {
                // Calls to getContextClassLoader from this class
                // never trigger a security check, but we check
                // whether our callers have this permission anyways.
                sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);

                // Fail fast
                sm.checkPermission(new RuntimePermission("setContextClassLoader"));
            }
            this.acc = AccessController.getContext();
            this.ccl = Thread.currentThread().getContextClassLoader();
        }

        public Thread newThread(final Runnable r) {
            return super.newThread(new Runnable() {
                public void run() {
                    AccessController.doPrivileged(new PrivilegedAction<Void>() {
                        public Void run() {
                            Thread.currentThread().setContextClassLoader(ccl);
                            r.run();
                            return null;
                        }
                    }, acc);
                }
            });
        }
    }

    /**
     * A wrapper class that exposes only the ExecutorService methods of an
     * ExecutorService implementation.
     */
    static class DelegatedExecutorService extends AbstractExecutorService {
        private final ExecutorService e;

        DelegatedExecutorService(ExecutorService executor) {
            e = executor;
        }

        public void execute(Runnable command) {
            e.execute(command);
        }

        public void shutdown() {
            e.shutdown();
        }

        public List<Runnable> shutdownNow() {
            return e.shutdownNow();
        }

        public boolean isShutdown() {
            return e.isShutdown();
        }

        public boolean isTerminated() {
            return e.isTerminated();
        }

        public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
            return e.awaitTermination(timeout, unit);
        }

        public Future<?> submit(Runnable task) {
            return e.submit(task);
        }

        public <T> Future<T> submit(Callable<T> task) {
            return e.submit(task);
        }

        public <T> Future<T> submit(Runnable task, T result) {
            return e.submit(task, result);
        }

        public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) throws InterruptedException {
            return e.invokeAll(tasks);
        }

        public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
                throws InterruptedException {
            return e.invokeAll(tasks, timeout, unit);
        }

        public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
                throws InterruptedException, ExecutionException {
            return e.invokeAny(tasks);
        }

        public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
                throws InterruptedException, ExecutionException, TimeoutException {
            return e.invokeAny(tasks, timeout, unit);
        }
    }

    static class FinalizableDelegatedExecutorService extends DelegatedExecutorService {
        FinalizableDelegatedExecutorService(ExecutorService executor) {
            super(executor);
        }

        protected void finalize() {
            super.shutdown();
        }
    }

    /**
     * A wrapper class that exposes only the ScheduledExecutorService methods of a
     * ScheduledExecutorService implementation.
     */
    static class DelegatedScheduledExecutorService extends DelegatedExecutorService
            implements ScheduledExecutorService {
        private final ScheduledExecutorService e;

        DelegatedScheduledExecutorService(ScheduledExecutorService executor) {
            super(executor);
            e = executor;
        }

        public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) {
            return e.schedule(command, delay, unit);
        }

        public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
            return e.schedule(callable, delay, unit);
        }

        public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit) {
            return e.scheduleAtFixedRate(command, initialDelay, period, unit);
        }

        public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay,
                TimeUnit unit) {
            return e.scheduleWithFixedDelay(command, initialDelay, delay, unit);
        }
    }

    /** Cannot instantiate. */
    private Executors() {
    }
}
